when do I pronounce the "s" in plus?

Bonjour Mary !

In general (meaning there are exceptions!), you will pronounce the "s" in "plus" when it is used in a positive way, meaning "more", and the "s" will be silent when "plus" in used as a negation ("anymore", "any longer").
"J'ai pluS de billes que toi." (I've got more marbles than you.)
"Je ne veux plus [mute] la voir." (I don't want to see her anymore.)

Of course, as always in French, sometimes the "s" can also be silent in positive contexts, for euphony reasons (it sounds better).
"Il a plus de mille livres." (He has more than a thousand books.)
However, you will never pronounce the "s" in the negation "plus"!

I hope that's helpful!
À bientôt !